1. Field of the Invention
This invention relates generally to the field of absorbent consumer paper products, such as absorbent paper towels. More specifically, this invention relates to an improved method and system for embossing a pattern on an absorbent paper web that is less likely to cause the web to burst during embossing than was previously thought possible.
2. Description of the Related Technology
Absorbent consumer paper products such as the high quality paper towels, napkins and toilet tissue manufactured by Scott Paper Company are in wide use throughout the world.
In manufacturing such products, an absorbent base sheet is commonly embossed to increase the bulk of the product, improve product absorbency and roll building characteristics, and to create an attractive pattern on the product before it is packaged. Embossing can also aid in securing superposed plies of the absorbent web together. The standard embossing process in this industry involves passing the base sheet between a pair of rollers, at least one of which has a pattern thereon that is intended to be impressed onto the base sheet.
Unfortunately, stresses created in the base sheet during the embossing process can cause the web to rupture at certain stress concentration locations in a manner that the inventors hereof refer to as "bursting." Certain base sheet materials are more susceptible to bursting than others. For example, base sheets that have been re-creped (Single Re-Crepe or "SRC") or double re-creped ("DRC") present a relatively low risk of bursting because the re-crepeing process adds strength and stretchability to the base sheet. On the other hand, a heavy wet crepe base sheet such as that used in Scott Paper Company's SCOTTOWELS brand of paper towels is quite susceptible to bursting. In addition to the type of base sheet, the type of embossing pattern is an important factor in determining whether or not bursting occurs, because it is the embossing pattern that will determine where the locations of stress concentration will be and the intensity of stresses that will be created at each location. For example, embossing patterns that define pillow areas with continuous, substantially unbroken outer borders will create higher stresses within the pillow areas than patterns with broken borders because the tension created during embossing must be absorbed entirely within the pillow area, and cannot be passed onto any adjacent area. In addition, patterns with smaller pillow areas are more likely to cause bursting than larger pillow area patterns, because there is less area in a smaller pattern to absorb the tensions that are created during embossing. For example, for a heavy wet crepe type base sheet having a cross-direction stretch of 7% or less, the inventors have found that bursting is likely to occur. There is a category of pattern-material combinations, then, that the industry has not been able to achieve at consumer quality standards because of bursting problems. Unfortunately, many patterns that would otherwise be attractive to consumers are included in this category.
FIG. 1 is a photograph of a heavy wet crepe base sheet that has been embossed with a pattern of continuous intersecting lines to define diamond-shaped pillow areas. A typical heavy wet crepe base sheet may be defined as having a basis weight of 30 lbs/ream, a machine direction dry strength of 60 oz/in, a machine direction stretch of 11 percent, a cross direction strength of 45 oz/in, a cross direction stretch of 5 percent and a bulk of 200 (caliper of one sheet in mils.times.24,000). Clearly visible in FIG. 1 are a number of fractures in the base sheet that were caused by bursting during the embossing process. As may be seen from the photograph, bursting creates an unattractive blemish on the product, and weakens the product for consumer use.
It is clear that a need exists in this area of technology for a solution to the bursting problems that plague certain combinations of embossing patterns and base sheet materials.